A phase-locked loop is widely used in technical fields, such as frequency control, frequency synthesizing, FM (frequency modulation) demodulation, data recovery, signal synchronization, etc. One of the applications of the phase-locked loop is a jitter attenuator for removing jitter which is fluctuation of phase carried on a clock signal.
A first conventional phase-locked loop includes a phase/frequency detector for comparing phases between an input signal and an output signal, a loop filter for supplying a control signal in accordance with a signal supplied from the phase/frequency detector, and a voltage controlled oscillator. The voltage controlled oscillator adjusts a phase of the output signal in accordance with the control signal supplied from the loop filter in order that the input signal and the output signal are synchronized.
When a phase-locked loop is used for a jitter attenuator, it is necessary that a loop bandwidth thereof is narrower than a frequency component of jitter to be removed. Therefore, a phase difference between an input signal having jitter and output signal having no jitter becomes large. In such a phase-locked loop, it is necessary that a tracking range is wide in order to keep synchronism between an output signal and an input signal without phase slip.
Such a phase-locked loop having a wide tracking range has been proposed in a report "JITTER ATTENUATION PHASE LOCKED LOOP USING SWITCHED CAPACITOR CONTROLLED CRYSTAL OSCILLATOR" IEEE 1988 CUSTOM INTEGRATED CIRCUIT ITS CONFERENCE.
Next, a second conventional phase-locked loop shown in the above report will be explained. The second conventional phase-locked loop includes, in addition to the first conventional phase-locked loop, first and second frequency demultipliers for dividing frequency of input signal and output signal into a predetermined number (N).
According to the second conventional phase-locked loop, the divided signals are compared by a phase/frequency detector, so that tracking range of the phase-locked loop is expanded.
However, loop gain and loop bandwidth are decreased, and pull-in time is long. Further, jitter is increased at low frequencies of the output signal.